


Ideal Numbers

by winter_machine



Category: Grey's Anatomy
Genre: F/M, Library Sex, Maddek - Freeform, Mark/Addison/Derek, Medical School Flashback, Multi, OG Trio, OT3, Shameless Smut, Threesome, Threesome - F/M/M, Two Thirds of My Stories are Currently Threesomes
Language: English
Status: Completed
Published: 2018-02-25
Updated: 2018-02-25
Packaged: 2019-03-23 15:53:12
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 5,775
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/13791042
Author URL: https://archiveofourown.org/users/winter_machine/pseuds/winter_machine
Summary: Just Mark, Addison, and Derek innocently studying in the library back in their youthful medical school days.  Addison is wound up about their upcoming exams. Mark has an idea. Derek is game.





	Ideal Numbers

**Author's Note:**

> OT3 4ever. Mark, Addison, and Derek didn't have nearly enough threeway sex on the show. Luckily, they've had plenty of it in fandom. Here's some more. Rated Explicit to stay on the safe side (as one should), but open to suggestions.

**Ideal Numbers**

“Addie … are you okay?”

She has to bite back a sarcastic response because their first exam is tomorrow.  _Tomorrow._ How is she supposed to be okay? Last year she was a physics major dreaming about medical school, and this year she’s actually here, in the library, about to face down her first year-end exam.  Naturally, it’s Molecular Mechanisms, the class that’s given her the most run for her money.

She sighs.  “I’m fine.”  She doesn’t want to be rude.  Not to Derek, who is by all accounts the perfect boyfriend.  From the moment their eyes met across their cadaver on the first day of Gross Anatomy, she was smitten.  (She made him ask her out three times before she said yes – she has _some_ pride – but in truth, she was ready from their very first meeting.)

Then she made him wait another three dates before …

Well.  They’ve been dating all year, and they’re close and it’s _good._ It’s really good.  She’s more experienced than Derek, who rather sweetly dated the same girl all four years of college while Addison tried out her new, no-braces, no-lisp, no-band-uniform power on quite a few more than a handful, but he’s pretty much a natural. He’s smart and funny and he _gets_ her. 

(And yeah, he comes as a package deal with his annoying childhood friend, but Mark isn’t _that_ bad.  He flirts with anything that moves and he hasn’t met a double-entendre he doesn’t like and he’s incapable of dating fewer than three girls at any one time, but he takes surprisingly good notes in class and he’s always willing to share, even with girls – like married, super-religious Shoshanna – who are never going to sleep with him.)

“Really? ‘Cause you seem wound up to me,” Mark chimes in.

Right.  And he’s here, because Mark’s sexual escapades have already gotten him kicked out of two study groups, so Addison relented and let him join theirs at the last minute, hoping Sam and Naomi wouldn’t be too mad.  Five is a _lot._   Everyone knows four is the ideal number for a study group.  Any fewer and you don’t get enough viewpoints; any more and you don’t get enough airtime. 

But when she finally agreed, Derek was so happy that it was practically worth it.  Derek is, without a doubt, the best guy she’s ever dated.  He’s good-looking, but not in that snotty way that drove her crazy with Archer’s friends.  He’s driven, but not so competitive that he’s mean about it.  And he’s just plain sweet, doing stuff like carrying her books when it’s icy out, bringing her chicken soup when she’s sick, and saving the best carrels for her in the library.

Like this one. 

It’s private, with transem windows set high on the white-painted walls, a large study table big enough for a group, and padded pleather chairs with sturdy wooden arms to cushion hours and hours of review.  It’s twice the size of most of the carrels; it can easily fit four and, Addison supposes, it will just have to expand to fit five.

“Are you sure?” Derek touches her shoulder. “You’re shivering.”

That’s because like the rest of the library, the study carrel is chilly even in humid mid-May, air-conditioning pumping through the vents.  The lightweight summer dress she wore, loose and flowery, with espadrilles – the lightest thing she owns – is no match for the climate control, and even with a cardigan on top, gooseflesh is rising on her thighs, making her cranky.

“It’s just cold in here,” she says.  “But that’s good because it will keep us awake.”

Derek frowns. “When was the last time you slept?”

“Last…” Her voice trails off.  Derek crashed at her place last night – his preferred mode of sleepover, since Naomi makes killer waffles – so he’s well aware she was up until dawn pacing and making notecards.

“I’ll sleep after exams,” she says hastily.  “So, are we studying or not?”

 Mark is studying _her_ for some reason.  “You’re wound up, Addison, just admit it.”

“Fine, I’m wound up!” She tosses her highlighter down.  “Exams start tomorrow, there’s a heat wave but it’s freaking freezing in here – how am I supposed to _not_ be wound up – Mark, what are you doing, I just said I was cold…?” Because he’s peeling her white cardigan away from her shoulders and sliding it down her arms.

She sees Mark and Derek exchange a look.

“Winding you down,” Mark says with a smile.

Before she can respond, his fingers are digging into the sore spots on her tense shoulders, releasing stiff muscles.

She glances at Derek uncertainly.  Everyone knows Mark is a player; Addison’s actually witnessed him moving right from friendly backrub into high-speed seduction, so this is … weird.  But Derek has pulled out the chair opposite Addison’s and is just sitting there serenely, smiling at her in an encouraging way. 

“You do need to wind down, Addie,” Derek says warmly.  “So enjoy it.”

It is, she has to admit, a great massage.  Whatever Mark’s been doing with every other girl in their class, including the one who threw all his books out the window of her fourth-floor dorm and the one who slapped his face in front of their entire Fundamentals of Clinical Medicine class, he’s obviously learned some things.  Her muscles fairly melt under his hands and she’s starting to think this wasn’t the worst idea in the world.

Then his fingers slip under the collar of her dress, brushing the sensitive base of her neck, sliding onto her bare shoulder...

“Mark.” She turns abruptly.  “What do you think you’re doing?”

“I told you,” he says simply. 

Addison glances at Derek.  Hopefully he won’t be angry enough to throw a punch at his best friend.  “Well, it’s totally inappropriate, Mark, because – “

And her voice trails off because Derek is crouching in front of her, suddenly, resting a hand on her bare thigh just below the flowing hem of her dress.  He doesn’t look angry at all.  His blue eyes are – okay, they’re ridiculously beautiful, but other than that they look calm, a little hazy …

She’s confused.

“It’s okay, Addie,” Derek says gently, his warm palm circling the chilly flesh of her thigh.  “We just want you to feel better.”

“I feel okay,” she says weakly.

“But you’re wound up.  And that’s bound to affect your performance on tomorrow’s exam.”

She opens her mouth to respond but for some reason a sigh escapes instead, maybe because Mark’s hand has slipped under her collar again, and Derek’s hand is gliding down her leg, tracing the shape of her calf, then back up again.

“We can stop,” Derek says firmly.  “Whenever you want.”

“Want us to stop?” Mark asks now, and his fingers dip a little lower on her shoulder.

“Well, you can, um … you don’t have to stop,” she mutters, glancing past Derek’s curly head to the pile of books on the table. 

“I was hoping you’d say that.” Derek grins at her, then leans forward to kiss her deeply. Her fingers rise automatically to tangle in his thick hair and his hands are stroking her own long hair, and she’s enjoying the closeness when she yelps into his mouth.

Someone is palming her breasts, and it’s not Derek because both his hands are in her hair. 

Mark raises both his hands innocently.

For some reason, she misses the contact.  It’s just that it’s cold in here, and the warmth of his palms was keeping her from freezing to death.  Very slightly, making sure to maintain her dignity, she angles her shoulders toward Mark.

He catches her intent immediately and raises his eyebrows, but he relents quickly and his hands return to her body, gentle and obviously practiced, just massaging her.  Derek is still kissing her, long, slow kisses, the kind that take her breath away, and stroking her neck; her breath catches when she feels Mark’s thumb brushes her nipple just as Derek sucks her lower lip into his mouth. 

She draws back, panting slightly, and smiles at them almost shyly. 

“You okay?” Derek asks, cupping her cheek. 

“I’m okay.”  She lowers her eyes. “I’m still … a little wound up, though.” 

When she looks up both guys are grinning. 

Then Mark is turning her chair so she’s facing away from the table, and both of them are crouched in front of her.  She shrinks back slightly, instinctually, because the predatory looks on both their faces are making her feel…

…okay, fine, they’re making her feel incredibly turned on.  Still, she smooths her skirt primly over her legs.  She’s not averse to … whatever this is, but she likes to lead.  “What we should do,” she says bossily, “is that one of you can – oh!”

She’s lost the plot again, because they’ve somehow slid her down the chair enough that one of her legs is slung over each of the armrests, opening her thighs to the chill of the air conditioning. 

“What are you …”

Mark’s head disappears under her skirt and then it’s Derek’s hands on her shoulders, massaging her, soothing her, while the rough stubble on Mark’s face burns her thighs.

Her cheeks burn when he presses his face between her thighs and inhales deeply.

“Mark!”

“Just making sure you’re enjoying this,” he says innocently, his voice slightly muffled by her skirts. 

She looks up at Derek for sympathy, because Mark is so _vulgar_ , but Derek is just smiling at her, his blue eyes hooded with lust.  He presses his lips to her neck, then, and down to the ultra-sensitive spot behind her ear that once made her knee him in the groin. 

That night was a bust, and she felt terrible about it, but then they knew the power of that spot and knew to tread carefully.

Derek must have said something to Mark because while her boyfriend is massaging her shoulders and tickling her neck with his lips, Mark’s hands have come up to hold her thighs in place. She won’t be kicking anyone, thank goodness, but pleasure swirls inside her.  Mark is pressing heated kisses to the inside of her thighs, and it shouldn’t feel so good, but she finds herself sliding down the chair. 

They hoist her back up together, and then Mark’s fingers slide into the silky fabric between him and her pleasure. 

She squeals before she can stop herself, then slaps a hand over her own mouth.  Derek laughs into her neck.

“I told you she was loud.”

“Dude, we’re roommates, you didn’t _have_ to tell me.”

Addison is blushing furiously, but she has to time to argue because Mark has a handful of silk and he’s ripping it off her in a way that should make her furious but instead results in a humiliating rush of moisture at the center of her body.

“Here.” Mark tosses the discarded panties to Derek, who tucks them neatly into the back pocket of his jeans.

She’s still wearing her flowery sundress – it’s modest, scoop-necked, with a bodice of decorative buttons and a loose a-line skirt that makes it all too easy for Mark to basically camp under as he trails soft kisses and rough stubble in turn along her sensitive flesh.  An ache for more is spreading in her stomach and Mark glances a palm on her clenching muscles, pops his head out, and smirks.  He’s looking right at her, an insolent glare that throbs right between her legs, when he flicks his wrist and she shrieks before she can stop herself.

“Oh, my god.” She covers her mouth with both hands, then spreads her fingers so she can talk.  “Someone’s going to hear us!”

“Not if you’re quiet,” Derek assures her, slipping his hand into the bodice of her dress and rolling a nipple between his fingers.

“How can I be quiet if you’re –“ her words turn into a groan as Derek sucks the same nipple into his mouth and even through two layers of fabric it goes straight to her center.

“Fine,” she pants.  “Keep going.”  She covers her mouth again with both hands, and when Mark slides his fingers inside her – two, and they’re not small fingers – her squeal is muffled by the skin of her palms.  He’s curling his fingers, beckoning her, but ignoring the spot of need that she’s dying for him to touch.

Then his tongue swirls around it, just once, and she yells into her palms, praying that nobody is walking by the study carrel.  The window is high enough that they would just see – well, her head and Derek’s, and she’s still wearing her dress, so…

“Mark!” She yelps his name from behind her hands as his tongue draws a figure eight and his fingers pump in and out of her in a rhythm that she can’t quite anticipate.  Which means she can’t be prepared.

Which means she has to cover her mouth again.  Hard.

Mark’s head pops out from under her skirt.

“Hold her hands,” he instructs Derek.  “Make her work for it.”  To Addison, he raises one finger to his lips in the universal gesture for _shh_ , then dives back underneath her skirt.

Derek’s fingers circle her wrists, pulling her hands away from her face, and he’s kissing the sensitive undersides of her arms and stretching them up, over her head.  Addison wants to protest but it’s hard to form words right now because Mark’s fingers are moving inside of her in a way that makes it feel like he’s everywhere at once, and then his lips are on her again, and if she weren’t trapped by his arms and the chair and Derek she’s pretty sure she’d be hitting the ceiling right about now.

A moan escapes her and she reaches to clap her hand over her mouth, forgetting that Derek’s holding both of them.

“Addison,” Derek scolds from above her head, sucking one of her fingers into his mouth in a way that goes straight through her body, “this is a _library._   You need to keep your voice down.”

There’s no sense pointing out that it’s entirely his fault, right?

Well, actually more like half his fault, because Mark’s sandy head is still buried between her thighs.  The strain of not making any noise is going to kill her.  And they are going to have a _lot_ of explaining to do to the dean.

“Oh!” She inhales sharply, trying not to cry out, but the pressure building under Mark’s tongue is too much.  She opens her eyes with some effort to see Derek holding both her wrists and smiling down at her, clearly enjoying her struggle far too much.  She shifts in the chair, needing to feel Derek’s hands on her too.  He reads the question in her pleading eyes and responds with one of his own.

“Will you be good?” When she nods shakily, he draws both her hands over her head, letting them rest loosely behind her neck.  “Keep them there,” he warns, and kisses a trail down her arm, across her collarbones, along her throat, while his hands stroke her through the thin fabric of her dress.

Then Mark is slowing down and she doesn’t know why, but she wriggles against his face, desperate for more pressure, and feels him laugh into her heated flesh.

“So demanding,” he teases, and he withdraws his fingers – _damn it_ – and strokes the inside of one thigh with an agonizingly light touch. 

“Please,” she adds, and this time Derek laughs.

“You can’t say no to Miss Manners.” 

“True.”  Mark leans back and the sight of his slick face makes her swallow hard.  For a brief, wild moment the import of what they’re doing hits her.  They’re medical students, for crying out loud, and this is a _library_ and she’s a lady, not someone who … does dirty things in libraries.

“Actually, you _are_ someone who does dirty things in libraries, Addie.”

Oh, crap. She didn’t realize she’d said that aloud.  Or maybe they can read her mind now because they’re so close, all three of them, so close that she can feel Mark’s warm breath on her thighs and Derek’s against her neck, his fingers toying with a stiff nipple through the fabric of her dress.  He knows exactly how to tease her and he seems to like the way Mark is watching her squirm. Mark has leaned back on his haunches, and Addison desperately misses the contact, bucking her hips as much as she can in her limited position.

Mark laughs at her and she scowls.  Leaving her hanging like that is just bad manners, and when her most seductive gaze doesn’t inspire to him to get back there and finish what he started, she makes up her mind.

She’ll do it herself.  Isn’t that what they say, if you want something done right … to do it yourself?

One hand slides down from where Derek propped them earlier, and her boyfriend and his best friend both watch her fingers move across the flowery fabric of her dress, over the crumpled skirt, and skate across heated flesh, almost exactly where she-

“Ow!” She gasps in shock when she feels a palm come down across her thigh.

“Didn’t Derek say not to move your hands?” Mark’s tone is mild, and he’s smiling at her.

She gapes. The sting abates quickly, warmth flooding the area instead.  Still, it was … _loud._ “Someone will hear us!” she hisses.

“Then I guess you should behave yourself,” Mark shrugs, and plants a kiss where he marked her with his palm. 

Derek points meaningfully at the top of the chair and Addison sighs, bringing her free hand back to join the other behind her neck. 

Meanwhile, Derek’s somehow undone all the tiny buttons on her bodice – she thought they were for show when she bought it – and his familiar hands are under her back, lifting her so that her breasts, already spilling out of their lace cups, practically leap into his mouth.  With Derek supporting her upper body and Mark its lower half, she feels utterly weightless.

Not just weightless.  _Flying_.

“She seems less wound up,” Mark observes casually as she spasms around his fingers.  “Good work, Shep.”

“Not bad yourself, Sloan.” Derek’s words are slightly muffled by the nipple that’s still between his lips.

As soon as she’s able to speak again, Addison does: “When you two are done congratulating yourselves – oh!”

Because Mark has lifted her up so quickly and smoothly that she has almost no time to be startled before he sits down again, switching their places so he’s the one in the chair and she’s on his lap, facing him, the rough denim of his jeans exquisite agony against her bare thighs.  But the wooden arms of the chair keep her from straddling him completely, and she shifts, looking for purchase. 

“Let me help,” Derek offers, and the next thing she knows her legs are stretched up, up and over Mark’s shoulders and it’s too much, she shouldn’t be this flexible, but somehow it’s perfectly comfortable, like she was born to do it.

“Derek…”

She glances at her boyfriend. She’s not sure she could stop if she wanted to, but it seems like they should at least both be on board for this part.

He’s looking at her with lust in his eyes, not a hint of jealousy.  “Go ahead,” he says.  “Show Mark what he’s been missing.”

And he’s not just approving, he’s _helping_ , because in a rush of metal, belt and zipper and buttons, Mark is springing free and then she’s sinking down on him with a breathless cry as Derek guides her route.   He’s deep inside her almost immediately and her muscles shift, stretching around him.  Derek strips the flowery dress over her head – it feels like a formality at this point anyway, hanging half off her body – and positions himself behind her, sometimes lazily circling a pebbled nipple, sometimes weighing the shape of her breast in his palm, sometimes holding her waist to help her move faster.  Her head tips back as the pressure inside her builds again.  Mark is close, she can feel it, even though she’s never touched him before, never felt the hard length of him inside her or the wide span of his hands on her hips.

She feels Derek pause, one hand stilling on her breast. “Did you hear - is that someone coming?”

Addison freezes with panic and then feels the moment the clench in her muscles drives Mark over the edge. 

“Derek!” Addison scowls, or scowls as much as she can when Mark is resting from his exertion by licking a trail across her collarbones.  “You made that up?”

“Not really,” Derek smirks.  “I mean, someone _was_ coming.”

Addison reaches out to swat him but he catches her hand, lowering it to the bulge in his jeans.  She cups him through the denim, still glaring, but also loving the effect she has on him when she sees the smoky blue color of his eyes.  He can’t be comfortable, not like that, and she makes short work of the fastening of his jeans.  She’s limp with pleasure, sticky and sated, and the least she can do is return the favor.

Mark doesn’t seem to want to let go of her, though; he’s softened inside her but seems perfectly comfortable with her weight on his lap, her legs circling his shoulders; he turns his scratchy face, alternating between kissing her thighs and lightly tugging on her nipples with his way too talented lips.  Ugh, no wonder he’s so cocky. Apparently it’s not all in his head.

“Mark, didn’t your mother teach you to share?”

“Nah, she was never around,” Mark says cheerfully, smiling at Derek while he rolls a nipple between his fingers, making her moan. “ _Your_ mother, on the other hand….”

“Moving on,” Derek interrupts hastily, “before you kill the moment.”

Mark is palming one of her breasts now, lips and tongue caressing the other one, and he shows no sign of stopping.  Her head drops back, long damp hair tickling the small of her back.  Derek sweeps a handful of her hair into his fist and tugs lightly.

“Remember me?”

But he doesn’t sound mad, just affectionate, and she giggles in response, reaching for him; he obliges with a long, deep kiss as Mark teases the flesh of her breasts, blowing softly against her heated skin.

Mark starts to ease her off his lap, then seems to think better of it, squeezing his hand around soft flesh and kissing the side of her neck, hard.  She winces, but it’s painful just for a second and then it’s pleasure – broad, sweeping pleasure.

“Something to remember me by,” he teases, but the look in his eyes is dark, intense, anything but a joke. He marks her two more times; she whimpers with each one, and each time he soothes her skin afterwards with a gentle caress.

“Before we graduate, Mark,” Derek cuts in, and then her boyfriend’s familiar hands are at her waist, lifting her off Mark’s lap.  He pulls her flush against him, rocking their pelvises together, and Addison cries out at the press of metal and rough denim from his open fly, then sighs at the pleasure of warm, satin-covered steel pressing against her sensitized flesh.

She moans his name into his neck, lifting one of her legs to wrap around him.

“One sec,” Derek says.  Gently, he peels her off him, taking a moment to admire the view, and then he raises a hand above the study table.

“I’ve always wanted to do this,” Derek muses, and he sweeps the contents of the table onto the floor.  Notecards, pens, highlighters, and a half empty bottle of water fall to the industrial carpeting.

“Derek!”

Mark lifts her onto the desk.  “Nice thinking.  It’s the perfect height.”

She’s completely naked at this point, no idea where her panties are – oh, that’s right, Derek slipped them into his pocket at one point.  One of them threw her bra … _somewhere_ , but she’ll have to figure that out lately.  Right now it’s just survival, it’s just breathing through the waves of sensation they seem determined to throw at her.  Sweat slick on the desk, she slides and they catch her, pinning her to the slippery wooden surface flush with the edge. 

Mark is kissing her now, tongue swirling in her mouth, and she tastes herself – faint, like honeysuckle, not entirely unpleasant, and then Mark is gone and her mouth is stretching wider to accommodate Derek, to engulf the familiar sensations of his straining flesh  

“Did you think it was only the Addison show?”

Derek grins down at her, stroking her cheek.  Mark is next to him, running his fingers up and down her body, slowly, lightly, seemingly with no real purpose unless it’s to soothe her enough that the throbbing warmth in her mouth slides deeply into her relaxed throat.

“Jesus.” Derek looks like he’s wobbling on his feet, and out of the corner of her eye Addison sees Mark steady his friend. But mostly she’s watching Derek, enjoying the way every sensation in his body plays across his face.  Then he’s stroking her hair, withdrawing carefully from her mouth, and she misses the way he filled her. 

He spins her easily on the sweat-slick table so that her legs are dangling off the end, her back supported by its surface, and his lips are trailing down her body.   

Then he slides deep and easy inside her and she gasps.  She’s going to feel this tomorrow, she knows, all of it.  She’ll feel it during the exam: the scratchy lace of her bra poking the pinched surfaces of her nipples, the seam of her jeans chafing sensitive flesh, the rashy prickles across her breasts, her throat, her shoulders, from unshaven kisses. 

How she’s supposed to concentrate tomorrow, she has no idea. 

She has no idea how to get through right now, much less tomorrow, because Mark has one nipple between his teeth; she squeaks without meaning to and he’s laughing into the swell of her breasts at her reaction.  Derek is moving slowly inside her, eyes hazy with arousal as she smiles down at her. 

Mark is leaning on the table alongside her body, stroking her stomach, her breasts, and then hovering his fingers close to where body is joined with Derek’s.

“I can’t … come again,” she murmurs, feeling she should warn him.  She flushes at saying the words, at being so direct.

“Says who?” Mark asks reasonably, and she feels the pad of his fingers as they trail down her body, along the trembling muscles of her stomach, and then stop.

Then he sucks a nipple into his mouth, warming it with the rough side of his tongue until she cries out.  He covers her lips with his, drowning out any further sound, before his fingers slide between her aching flesh and the slap of Derek’s hips.

She can’t make noise, can’t let loose the screams that want to escape but she also can’t _not_ react; she arcs off the desk, body electrified with sensation, and Mark pushes her gently back down with his free hand.  She sees Mark and Derek exchange a glance.

Derek slows his pace then, and she groans, wanting more, _faster,_ more.

And then Mark withdraws his hand too and she whimpers.

“You okay, Addie?” Derek asks innocently, still inside her, one hand lifting her thigh to spread her wide and the other bracing himself on the desk.

“Just … don’t stop,” she mutters, embarrassed.

Mark’s face comes into view over hers, looking rather pleased with himself.  “I thought you said you couldn’t come again.”

She glares.

“I guess she’s done, Derek.  Let’s pack it in.”

“No!” she cries before she can stop herself and hears them both laugh.

“Say it,” Mark challenges, and his fingers brush so lightly over her that she might have imagined it, except an obvious shudder runs through her body.  Damn it.

“I can come again,” she whispers, refusing to look them in the eye.  She feels the blush that’s been spreading up her chest creep into her cheeks. 

“Not _that_ ,” Mark says.

“That’s obvious,” Derek adds.

“Then what – oh,” she scowls. “You two are ridiculous.”

Derek raises his eyebrows.

“Come _on,_ you’re just being – wait!” she cries when she feels Derek start to pull out of her, immediately missing the feeling of him. 

“Yes?” Mark prompts.

“I was wrong,” she mutters.  “Fine, you happy?”

“Delirious,” Derek says, but to her relief his hips start to move again – too slowly and gently for her liking right now, but at least it’s something.

“Addison Montgomery, _wrong?_ Never,” Mark shakes his head.  “Why do you think we study with you?”

“It’s not for the perks?” she asks faintly.

Both of them laugh again. “I love a woman who can keep her sense of humor when we’re tormenting her,” Mark says matter-of-factly.  “Derek, everything you said about her is true.”

“What did he say about me?” Addison is honestly curious, even as the pulse between her legs threatens to drive her insane. 

Mark wraps a hand around the leg Derek isn’t holding and hoists it higher, so on his next slow, long thrust Derek goes even deeper, and she moans.

“That he’s in _love,_ ” Mark says it with a mixture of derision and affection.  “Isn’t that right, Shep?”

“Maybe.  Um. Yeah, I did say that,” Derek is blushing, and Addison rewards him by tightening her aching muscles in the way she knows he likes. 

“Derek, that’s so sweet,” Addison murmurs, genuinely touched.

“This is probably the first time in Columbia’s history that someone called filthy library sex _sweet,_ ” Mark observes. 

“Well, it’s filthy too,” Addison replies.  “Or it was, until you left me hanging.” 

“Fine.” Mark shrugs.  “I’ve always said three was a lucky number.”

Addison smiles with satisfaction, waiting for Mark’s talented fingers to slide between her body and Derek’s again. Instead, he disappears from view.

“What are you doing?”

“You’ll see.”

It turns out to be nothing more exotic than standing behind Derek’s pumping hips to brace Addison’s feet with his hands.

Except the simple change of position means that instead of just meeting Derek thrust-for-thrust she’s suddenly the fulcrum, the focal point, of the swelling energy among the three of them.  Derek speeds up at her semi-incoherent urging and with Mark bracing her he’s driven deeply enough that she can’t keep quiet anymore.

This time, no one tells her not to muffle her screams.

Mark’s hands are on her calves now, steadying her but also tilting her pelvis so that Derek is hitting her in exactly the right place.  She feels her eyes start to roll back in her head.  This pace is too fast, too deep, too much, no way he can keep doing this long enough except is it long enough or short enough because time feels strange right now like she’s drunk like she’s dreaming like she’s drowning and their voices are in her head, encouraging her, their flesh is on hers, teasing her, drawing it out, and she’s not going to be able to do this, she can’t, but then Mark releases her legs and suddenly he’s on the other side of her, he’s at her head, and he’s palming her breasts, his mouth on hers, and his tongue swipes hers just as Derek’s hips crash into her and then she’s screaming or he’s screaming or someone’s screaming in her mouth as everything explodes around her.

For a while afterwards, no one speaks.  Derek’s collapsed half on top of her, Mark on her other half, but each of them is supporting half his own weight on the other side, so she doesn’t feel crushed at all.  The sensation is warm and pleasant and perfect and she doesn’t want it to end as she fills her lungs with much-needed oxygen and waits for her quivering flesh to settle.

 “I don’t think I can ever move again.” Derek’s lips are moist against her neck when he speaks.

“I don’t _want_ to ever move again,” Mark adds against her chest.

She just smiles, drawing both her shaking hands up to rest one on each of her favorite study partner’s.  Five fingers work their way through Derek’s curls, five fingers scrape along Mark’s short, spiky hair and she sighs with deep, relaxed satisfaction

It takes a long time after that for their sweat-dampened bodies to withdraw from each other’s; they share sips of the discarded water bottle while their breathing slowly regulates.   

Then they settle back on the desk, Addison between Mark and Derek. 

“Cold?” Mark is running his fingers lightly along her arm, and gooseflesh rises in his wake.

“Not anymore.” She snuggles closer to Derek’s side, then pulls on Mark’s hand until he’s spooning her too – spooning both of them, kind of. 

“How are you feeling about exams now?” Mark asks, and from his tone she can tell he’s smiling.

“What exams?” She’s smiling too, into Derek’s shoulder, loving the feeling of his muscles under her cheek and the hard press of Mark’s body against hers.  She’s exhausted, and she’s sweaty, and she could really use a shower, and she’s lying on a (let’s face it) _defiled_ study table in the basement of the library but somehow it’s the most comfortable she’s ever felt.

Her eyes drift closed into blessed sleep.

..

 “Addison…”

“Mmph.”  She nudges the hand away.  Why are they bothering her, she just wants to enjoy the well-earned afterglow.

“Addie,” Derek says again. “Wake up.”

“Huh?” She blinks awake and sees a concerned-looking Derek through bleary eyes.  He’s leaning over her, holding an armful of textbooks. 

“You fell asleep on your notes,” he smiles at her. 

She rubs her eyes, trying to understand.

“And you drooled right here, on the pro-inflammatory cytokines _,_ ” Mark points out, and she sees Derek elbow him.

“What?” Mark asks defensively.  “She did!”

But Derek looks concerned.  “You okay, Addie? I think maybe you’re working too hard.”  He touches her cheek.  “You sure you want to study together tonight?”

“Of course.”  She runs her fingers through her hair, straightens her dress. 

“Well, Sam had to go to the lab; something happened to one of his samples, and Nai is helping him,” Derek picks up Addison’s cardigan and hands it to her, settling into the empty seat by her side.  “So it’s just the three of us.  That’s not the ideal number for a study group, Addie, I know.”

“It’s okay.”  Addison stretches slightly, working out the kinks in her neck, and smiles at Mark and Derek.  “You know, I think three might actually be the ideal number after all.”

**Author's Note:**

> This is why I don't ship, I thrip. Enjoy the OT3? Let me know...


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